Year 11 Chemistry: Chapter 7 :~ Forces between Molecules (Intermolecular forces)
7.4 Forces between Molecules
All the bonding forces covered so far (metallic, ionic and covalent) are
_______________ forces. They all involve the attraction of __________ charged
particles.
Water Molecule
 Demo: I will do this on Wednesday.
When you charge a plastic rod and place it near a small stream of water the water will
bend, suggesting the water molecules must have a __________ .
Polarised bonds
When two different non-metal atoms from a ___________ bond, one atom usually
attracts the bonding electrons more strongly than the other atom (electronegativity).
Example:
Hydrogen chloride (HCl – hydrochloric acid) Chlorine has an electronegative of 3.2
(table 7.4 pg 123) and hydrogen 2.1. Chlorine has the higher electronegativity therefore
the electron tends to spend more time closer to the chlorine atom.
H - Cl
This means the Chlorine end of the molecule becomes slightly negative and the hydrogen
end slightly __________ . The chlorine atom has a partial negative charge and the
hydrogen atom has a partial positive charge. (NOTE: this partial charge differs from ion
charges – electrovalencies).
*Draw in a section of Fig 7.23 (you must have 4 molecules of HCl bonded)
The covalent bond in the hydrogen chloride (H - Cl) molecule is said to be polarised and
the molecule itself is said to be ________ molecule. The molecule is also said to be
_________, because it has two charges ends or poles. The intermolecular force of
attraction is called a _________________ attraction.
Define: use the glossary
 Polarised bond: _________________________________________________
_______________________________________________________________
 Polar molecule: __________________________________________________
_______________________________________________________________
_______________________________________________________________
 Dipole: _______________________________________________________
_______________________________________________________________
 Dipole – dipole: _________________________________________________
Intermolecular forces and melting
Dipole-dipole attraction forces (intramolecular) are strong enough to hold hydrogen
chloride molecules together in a ________ lattice at ______ temperatures. These
forces are much ________ than the _________ bond _________ (intermolecular) the
hydrogen chloride molecule. Melting overcomes some of the forces of attraction, though
each molecule remains intact because of the strong covalent between the hydrogen and
chloride atoms (intramolecular).
The dipole-dipole forces between the molecules have _____ disappeared. They are still
present and hold the molecules to each other in the liquid state. Above melting
temperature and these forces do disppear…. but the covalent bond (intramolecular) is
still there holding the hydrogen and chloride atoms together.
Identifying polar molecules
Compare the two structural formulas on pg 124 Fig 7.25 (draw the figs in the blank
sections).
Both of these molecules have ______________ bonds. Formaldehyde is polar, the
oxygen side of the molecule is slightly negative and the hydrogen side is slightly positive,
therefore dipole!
Carbon dioxide molecule is systematical, it does not have __________ and
__________ sides. Both ends have a slightly __________ charge. The charges are
equal. Therefore the carbon dioxide molecule is not a dipole because the distribution of
charge is ______________ . It does have polarised bonds, but it is not polar and is
called non-polar.
Figure 7.26 (pg 124, please draw) is tetrafluoromethane it is a non-polar molecule,
although each of the covalent C-F bonds is polarised. It is sysmmetrical and therefore
non-polar.
For a molecule to be dipole:
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Polar molecules will attract each other with the ___________ side of one molecule
attracting the positive side of another molecule. This __________ dipole-dipole
attraction helps to old the molecules together in the polar covalent molecular solid.
Hydrogen bonding
Frozen lettuce and strawberries (also other fruits) become soft and mushy when they
thaw. Water ________ when it freezes and the ice crystals formed in the fruit damage
the _____ wall. The type of bond between the water molecules is particular important
and gives water its extraordinary properties. This force also holds the twin strands of
DNA together.
Hydrogen bonding is the most significant kind of dipole-dipole attraction.
*Draw in figs 7.27
Hydrogen bonding occurs between molecules in which hydrogen is bonded to
___________ , _____________ and __________. The hydrogen bonding between the
partially _________ hydrogen atom of one molecule and the lone pair on a
_____________ , ____________ or ___________ atom of a neighbouring molecule is
much _________ than other cases of dipole-dipole bonding.
Maximum attraction between the dipoles makes the water molecules space out as it
changes from a liquid to a solid. That is why ice expands and floats on water.
Hydrogen bonding is an _____________ force, the bonds are much than either
metallic, ________ or ___________ bonds (intramolecular).
Weakest bonding forces – Dispersion forces or Van Der Waals
The weakest bonding force operates in ____ substances. It arises from the constant
movement of ___________ in atoms and molecules.
Describe how dispersion force occur: ______________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
They are a weak force of attraction between all particles.
The effect of these forces can be observed in the melting and boiling temperatures of
the noble gases in table 7.5 (pg 126). The very low _______ and ________
temperatures indicate that the force between the molecules (intermolecular).
The melting and boiling temperatures of the noble gases increases as the size of the
atoms ___________ .The forces become more and more significant as the number of
electrons and so the size of the atom ____________ .
Dispersion or Van Der Waals are only considered when no significant stronger forces
exist. These forces are responsible for holding the molecules together in ___________
molecular substance. Examples where they exist, molecular oxygen, hydrogen, nitrogen,
all the noble gases and _________, in fact most Alkanes, Alkenes and Alkynes.
QUESTIONS: 8, 9ace, 10, 11, 12, 13, 14, 15, 21ab, 22, 23, 25.
READ: extension ‘Formulas for covalent molecular compounds’.
QUESTION: E3abc.
Handout Sheet ‘Covalent Bonding’ pages 16 – 20 will help you with the prac.
7.5 Covalent Lattices
Diamond, graphite are examples of _______ _________ lattices. Carbon dioxide is a
small covalent molecule. Carbon dioxide has __________ covalent ( _____molecular)
forces, though _________ dispersion forces (_______molecular). Therefore carbon
dioxide is a _____ at room temperature.
Diamond and _________ form ___________ bonds continuously throughout the
lattice. The structure of diamond is described as a ___________ lattice. The
structure of graphite is described as a ________ lattice. Theses giant covalent lattices
require exceptionally _______ temperatures to melt.
IN FACT DIAMOND, GRAPHITE AND CARBON DIOXIDE (IN SOLID FORM)
SUBLIME THEREFORE THEY DON’T MELT!
Define:
 Sublimation (use pg 128 not glossary) _______________________________
__________________________________________________________________
______________________________________________________________
Covalent Network Lattices
Diamonds (carbon) are the hardest naturally occurring substances known. Where can
industrial diamonds used:
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In diamond, each carbon atom is _____________ bonded to four others. These strong
covalent bonds are ____________ throughout the lattice. There are no weak links to
these lattices.
Silicon also forms a covalent network lattice, each silicon atom is bonded to ______
others.
In silica (silicon dioxide, SiO2 – also known as quartz) each silicon atom is covalently
bonded to _____ oxygen atoms and each oxygen atom is bonded to ______ silicon atoms
in the continuous, three-dimensional lattice.
Graphite: a Covalent Layer Lattice
Graphite (carbon) is very _______ in one direction but quite slippery and soft in
another direction. Graphite has a _________ structure. The carbon atoms within the
layers are held together by __________ bonds ( ______molecular) and are very
strong. The forces between ( ______molecular) layers are weak _______________
forces.
Within a layer, each carbon atom is bonded to three other carbon atoms. The fourth
electron is delocalised, therefore graphite is able to conduct electricity.
7.6 Using the Models of bonding
Using the models of bonding in 7.5, covalent molecules, covalent network lattices and
covalent layer lattices explain the below properties using their STRUCTURE.
Melting and Boiling Temperatures
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
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Electrical Conductivity
Covalent molecules have no charge therefore do not conduct electricity in their solid or
________ state.
In network lattices all electrons are ___________, therefore not free to move.
Substances like diamond that form network lattices do not conduct electricity.
In graphite one electron from each carbon is ___________ and free to move within the
layer. Therefore graphite does conduct electricity.
Hardness and softness
Diamond and other network lattices are usually very hard because ______________
________________________________________________________________In
graphite the forces between layers are ________ and so the layers can slide over one
another. Therefore graphite appears soft and greasy in one dimension.
Covalent molecular substances (eg carbon dioxide, methane) are often liquids or gases at
_______ temperature.
Chemical Reactivity
It is hard to ________ a strong bond. Substances composed of covalent network
lattices like diamond or silica do not react readily. Graphite is a covalent _________
lattice and is also very unreactive.
Small covalent molecules are mush more reactive. There chemical reactivity depends
however on the __________ and _________ of the bond.
Give two examples:
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QUESTIONS: 31, 33, 34.